To permit recording and analysis of structures of varying size using imaging methods, microscopes are equipped with various objectives that can be inserted, for example by being pivoted, into the optical beam path of the microscope, depending on the desired magnification.
If objectives are to be interchanged frequently, a plurality of objectives can be arranged in a holder, which permits fast and repeatable access to a selected objective and which allows for an objective interchange to be performed efficiently.
In what are known as revolvers, by way of example, a selected objective may be pivoted into the optical beam path of the microscope by rotating the holder about an axis of rotation. Focusing is achieved by way of setting a distance between the objective and a sample to be examined by displacing the holder having the objective or displacing the sample.
When it is displaced, the mass of the holder and of the objectives attached thereto causes high mechanical pre-loading of the drive train or the objective drive used for focusing, which disadvantageously helps bring about what are known as stick-slip effects, which occur in particular in guides and gears that are based on friction. In order to reduce disadvantageous mass-induced effects, complicated and therefore costly principles and constructive solutions must be implemented. In addition, relatively high drive capacities are required and the dynamic parameters of the output are unfavorable, which in turn has a negative impact on the positioning accuracy of the objective.
A high mass of the holder and of the drive train furthermore causes low natural frequencies and consequently high oscillation amplitudes at constant excitation energy. These have a negative impact on the resolution and the accuracy of the microscope in the case of great magnifications.
In addition, the eccentric centroid of the holder, in particular of the revolver, leads to additional and changing mechanical loads of the drive train. By way of example, due to the differing masses of the individual objectives, interchanging an objective results in a displacement of the centroid and causes a changed load situation as compared to the initial state before the objective was interchanged, which likewise has a negative effect on obtaining the image section and on the precision of the displacement motion.
International Application WO 2012/097191 A2 discloses a microscope that has an objective interchange apparatus having a holder for receiving a number of objectives at respective holder positions. When necessary, a selected objective is positionable in an optical beam path of the microscope in an objective receptacle that is configured for receiving and retaining the objective in a scanning unit. To this end, a scanning unit is displaced, together with the objective receptacle, to a specific holder position so as to lift the objective, which is located at the relevant holder position, with the objective receptacle from the holder and to connect it to the objective receptacle using a magnetic coupling. In addition, the objective is fixed at the objective receptacle by way of a locking mechanism based on movable balls.
After the objective is received, the scanning unit is displaced horizontally until the selected objective is situated in the optical beam path of the microscope. Disadvantageous here is the complicated horizontal displacement motion of the scanning unit and the necessary orientation of the scanning unit in the optical beam path of the microscope, which must be renewed for each objective interchange.